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Source: Journal - automotive materials and processes
Authors: Feng CE, Yan Bo, Guo Hongchang, Li Meixing, Wang Pei
Abstract: in order to improve the sealing performance of the protective cab of a heavy-duty off-road vehicle, the gluing process of the welding seam in the cab coating process was studied. Combined with the application of PVC weld sealant in coating production line, the composition, function and construction technology of PVC weld sealant are introduced. Based on the analysis of the structure and local characteristics of the protective cab, the paper designs different forms of lap structure of sheet metal and the gluing process of the key parts of the cab, and puts forward the problems that should be paid attention to in the coating process of the protective cab. At the same time, in view of the problems in the process of coating seam sealant, the corresponding solutions are analyzed and put forward.
Key words: protective cab welding seam coating seam sealant
01 foreword
Armor steel, as a traditional anti bullet material, is widely used in the field of armor protection. At present, the connection mode of armor steel structure mainly adopts welding technology, while the welding mode of thin armor steel protective cab mainly adopts continuous full penetration welding and partial intermittent welding. The structure of the protective cab is complex, the size is large, and there are a lot of intensive cross welds. Cab welding is a key process in the manufacturing of protective cab, which should reduce and prevent the generation of welding cracks and holes as much as possible. The welding quality has an important impact on the driving comfort, service performance and service life of the vehicle. In the process of welding, due to the influence of processing level, welding process and other factors, it is impossible to make absolutely perfect joint for the lap joint of sheet metal with different thickness. If the welding seam is not effectively sealed, air leakage, rain leakage, excessive noise and other phenomena will occur in the cab during driving.
In order to ensure the welding tightness of the cab and improve the waterproof, antirust, shock absorption and sound insulation performance of the cab, the cab needs to be sealed during the coating process. At present, most of the main engine plants of commercial vehicles in China use the wet touch wet coating process of PVC weld sealant and intermediate coat finish to seal the cab weld of trucks. The thickness of sheet metal in the protective cab is greater than that in the truck cab, and the lap structure of sheet metal is different. At the same time, the weld seam of the protective cab is dense, and the position of applying weld sealant is more than that of the truck cab. Therefore, it is necessary to apply glue and seal according to the structural characteristics of the protective cab. This paper mainly introduces the welding seam gluing process of the protective cab of a heavy-duty off-road vehicle.
02PVC seam sealant and function introduction
PVC sealant is a paste sealant made of emulsion or micro suspension polymerization of PVC resin, phthalic acid two ester plasticizer, diluent, tackifier, thixotropic agent, inorganic filler and pigment. It has good thixotropy. After heating and curing, it becomes a kind of elastomer with a certain bonding strength, which has a good binding force with electrophoretic primer, epoxy polyamide primer, etc. PVC weld sealant has good adhesiveness, flexibility, corrosion resistance and wear resistance, with low cost, tasteless, non-toxic and non-toxic in contact with skin. It is widely used in the sealing of inner and outer weld joints of cab BIW. Its main functions are as follows.
a. To improve the rain proof sealing performance of the cab, the cab is welded by major assemblies, and there are a large number of welds at the welding position. Applying weld sealant on the welds can improve the overall sealing performance of the cab.
b. To improve the sound insulation and noise reduction ability of the cab, the cab structure is complex, and there are many corner parts on the body, which are easy to be ignored in the process of cab welding and coating. The application of weld sealant in these areas can improve the local tightness and the sound insulation and noise reduction ability of the cab.
c. It has the function of shaping and modifying. Good PVC weld sealant will not flow on the weld. It can make the lap joint of sheet metal smoothly transition and play the role of leveling the body and modifying the surface of sheet metal.
d. To improve the antirust ability of cab, there are a large number of welds on the white body of the protective cab, the stress produced by welding is easy to focus on the heat affected zone of welding, and the corrosion resistance of the heat affected zone of welding is worse than that of other areas of the welding base metal. Therefore, the application of weld sealant at the weld can effectively block the contact between the corrosive medium and the weld, and improve the corrosion resistance of the white body.
The gluing process of 03 protective cab weld
3.1 cab coating process
At present, the cab coating of domestic commercial vehicles mainly adopts the coating process of wet contact between PVC weld sealant and finish paint. According to the preliminary test, the coating process of the white body of the protective cab is determined as pretreatment → electrophoresis → electrophoresis drying → stone impact resistant coating → asphalt rubber plate paving → grinding → seam sealant coating → intermediate coating → intermediate coating drying → grinding → finishing coat → finishing coat drying → finishing coat modification. Due to the good adhesion between PVC weld sealant and electrophoretic primer, PVC weld sealant is directly applied to the weld after electrophoretic polishing. After coating, the adhesive surface should be avoided. When the sealant is not gel, paint is directly painted on the driver's cab, so that the sealant on the weld surface and the paint on the surface of the cab are dried at the same time through the drying room.
3.2 gluing process of welds
For the construction of weld sealant on the coating production line, the high-pressure pump is mainly used to spray the glue which is extruded by the centralized glue conveying equipment, and the manual extrusion method is used locally. Different types and sizes of welds have different coating requirements, so different coating nozzles are needed. The nozzle mainly includes conical nozzle, circular nozzle, flat nozzle and L-shaped nozzle. According to the sequence from front to back, from top to bottom, from left to right, apply glue evenly along the weld seam with glue gun. After applying the glue, use the brush and rubber scraper to finish the gluing part. According to the different gluing position and gluing width, use a brush of different width to brush the seam sealant along the direction of the seam, so as to make the transition of the glue surface on the seam smooth and ensure that the gluing surface completely covers the seam. For the gluing parts with appearance requirements or assembly relationship requirements, rubber scraper is generally used to scrape the rubber end surface to ensure good appearance of the gluing end surface. After gluing, use porous cotton yarn cloth to wipe the excess glue around the weld to prevent the influence on the finish coating.
04 coating process requirements for welds of protective cab
The distribution, shape, size and thickness of weld sealant are affected by the local structural characteristics, lap form and thickness of sheet metal. Reasonable structural design is helpful to the application of weld sealant, and then to ensure the sealing of cab body.
4.1 lapping form and gluing requirements of major assemblies
Figure 1 shows two forms of butt joint of armored steel plate, and requires the overall dimension of the sealant applied. Figure 2 shows two types of overlap joints of armored steel plate, which are staggered and side-by-side. Fig. 3 and Fig. 4 are the fillet weld and T-joint weld of armored steel plate respectively, and the minimum radius of weld coating is 8 mm. For the sheet metal welding parts of each major assembly, all welds shall be fully covered by sealant during the coating process, and the coating thickness shall be 2-5mm.
Figure 1 butt joint of armored steel plate
4.2 lap form and gluing requirements of sheet metal at key positions
According to the structural characteristics and use requirements of the protective cab, the cab sealing mainly includes rain proof sealing and noise proof sealing. The rain proof sealing shall focus on the key structures of the cab top cover and front wall that may leak rain, and the noise proof sealing shall focus on the key structures of the cab rear wall and the lap of the floor sheet metal. The sheet metal lap forms of these key structures not only include four typical weld forms of butt joint, lap joint, fillet joint and T-joint, but also include the following forms.
Fig. 2 lap joint of armored steel plate
Fig. 3 angle joint of armored steel plate
Figure 4 T-joint of armored steel plate
4.2.1 installation frame of windshield
The joint between the upper bending plate and the lower bending plate of the windshield mounting frame shall have a flat surface, and the contact gap between the two bending plates shall not leak rain after coating. As shown in Figure 5, after the welding of the upper bending plate and the lower bending plate, apply the seam sealant, use a brush to smooth the sealant, and use a rubber scraper to scrape the upper glue surface of the weld, and finally wipe the excess glue near the gluing area.
4.2.2 external installation points of top cover
As other connecting parts need to be installed on the top cover surface of the cab, the top cover is easy to be washed by rain, and there are hidden dangers of rain leakage and corrosion in these parts. In order to prevent rainwater from entering into the cab, it is necessary to minimize the potential water leakage points and strictly seal the potential water leakage points. As shown in Figure 6, weld the blind hole nut seat on the inner side of the top cover, and evenly apply the weld sealant with a radius of 6-10 mm around the nut seat for sealing.
Figure 5 overlapping parts of bending plate of windshield mounting frame
Fig. 6 welding nut seat inside the top cover
4.2.3 door frame inner plate and other parts
As shown in Figure 7, the lap circle between the inner plate of the door frame and the outer plate of the side wall is easy to observe when getting on the train, where not only the appearance quality is required to be high, but also the lap joint between the inner plate of the door frame and the outer plate is required to be well sealed. Therefore, it is necessary to evenly apply the weld sealant at the lap joint to play a sealing role; at the same time, the rubber scraper shall be used to scrape the glued part to make the lap joint of sheet metal flat and play a role of shaping. Figure 7 overlapping part of inner door frame plate and outer side wall plate
05 main failure forms of weld sealant and preventive measures
Through the surface quality inspection of the small batch of cab after coating, the main problems were found as follows: local bubbles of sealant and local cracks of the joint between sealant and sheet metal.
5.1 bubbles produced by sealant
A small amount of bubbles will be produced after the middle coating and baking of the weld sealant, which will cause the local weld sealant to bulge, even affect the bonding strength between the sealant and the sheet metal, resulting in the poor sealing of the local sheet metal overlap. The causes of bubbles in sealant may be the change of sealant material, poor sealing of gluing equipment, air mixed in the pipeline, etc. according to the field production situation, the main causes of bubbles are air mixed in the process of applying weld sealant. There is a gap between the joint of sealant and sheet metal. After baking, air expands to produce bubbles. Therefore, when applying the weld sealant, the application pressure needs to be increased to ensure that the sealant is in full contact with the lap of the sheet metal to avoid cracks.
5.2 crack at the joint of sealant and sheet metal
After the middle coating is dried, the part of the joint between the weld sealant and the sheet metal does not fit tightly, and even cracks appear. Based on the observation and analysis of the on-site process and the cracking position, the main causes of the cracking are that the joint between the weld sealant and the sheet metal is not cleaned, there are dust and sundries, and the thermal expansion and cold contraction of the weld sealant and the sheet metal are not synchronized after the middle coating and drying, resulting in the cracking. Therefore, compressed air purging and other measures shall be taken to remove the sundries on the joint surface before applying the weld sealant.
06 conclusion
Taking the protective cab as the research object, the gluing process of cab weld in the coating process was studied. According to the structural characteristics of the protective cab, the coating process of the cab welding seam is designed. After the implementation of the coating process, the quality inspection after coating, and the tracking of field problems, the use of PVC welding seam sealant in the protective cab is verified, which provides a certain reference value for the promotion and use of PVC welding seam sealant in the protective body.
Source: Journal - Automotive Materials